The general stress response of Bacillus subtilis and close relatives provides the cell with protection from a variety of stresses. The upstream component of the environmental stress signal transduction cascade is activated by the RsbT kinase that switches binding partners from a 25 S macromolecular complex, the stressosome, to the RsbU phosphatase. Once the RsbU phosphatase is activated by interacting with RsbT, the alternative sigma factor, sigmaB, directs transcription of the general stress regulon. Previously, we demonstrated that the N-terminal domain of RsbU mediates the binding of RsbT. We now describe residues in N-RsbU that are crucial to this interaction by experimentation both in vitro and in vivo. Furthermore, crystal structures of the N-RsbU mutants provide a molecular explanation for the loss of interaction. Finally, we also characterize mutants in RsbT that affect binding to both RsbU and a simplified, binary model of the stressosome and thus identify overlapping binding surfaces on the RsbT "switch."